The 2016 Central Italy Seismic Sequence: A First Look at the Mainshocks, Aftershocks, and Source Models

The 2016 central Italy seismic sequence consists so far of a series
of moderate-to-large earthquakes activating within a fewmonths
along a 60-km-long and Apenninic-trending normal-fault system.
Regrettably, the high vulnerability of the local infrastructure
and the shallowness of the largest events (depth around 8 km)
resulted in 299 casualties and more than 20,000 homeless, with
great difficulties in the disaster management. The sequence
evolved around its largest event (Mw 6.5, 30 October) that occurred
right in the middle of a fault system already activated two
months before with a first Mw 6.0 mainshock (on 24 August)
located to the south near the town of Amatrice. Then, another
Mw 5.9 mainshock occurred just four days before the largest
mainshock (26 October) at the northernmost extent of the sequence,
near the town of Visso.We analyze the space–time evolution
of the first four months of seismic activity through the
relocation of ∼26; 000 earthquakes and the kinematic source
models of the three mainshocks. All the main events nucleated
at the base of a southwest-dipping normal-fault system segmented
by the presence of crosscutting compressional structures.
The presence of these inherited faults, separating diverse geological
domains, appears to modulate evolution of the sequence interfering
with coseismic slip distribution and fault segments
interaction. Several secondary antithetic and synthetic faults
are located at a shallow depth (<4 km), both in the hanging
wall and footwall. The whole normal fault system, confined
within the first 8 km of the upper crust, is bounded below
by a shallow east-dipping and 2–3-km-thick layer in which small
events plus a series of large extensional aftershocks (≈Mw 4)
occur, possibly decoupling the upper and lower crusts.